The present disclosure relates to a power amplifier module. A mobile communication terminal such as a cellular phone includes a power amplifier that amplifies a radio frequency (RF) signal to be transmitted to a base station. The power amplifier includes an amplifier that amplifies the RF signal, and a bias circuit that controls the bias point of the amplifier. A bias circuit of this type that is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2014-171170 is known. The disclosed bias circuit includes an emitter follower transistor that supplies a bias signal to an amplifier, and a constant voltage generating circuit for generating a constant voltage to be supplied to the collector of the emitter follower transistor.
When a bipolar transistor in an amplifier is driven with a base-emitter voltage of the bipolar transistor at a fixed voltage, a collector current increases in accordance with an increase in temperature. The increase in collector current causes an increase in power consumption, which increases the temperature of the bipolar transistor. Accordingly, the collector current further increases. That is, thermal runaway occurs. In a circuit configuration of the related art in which a constant voltage is supplied to the collector of an emitter follower transistor that supplies a bias signal to an amplifier, it is difficult to suppress the increase in collector current caused by thermal runaway, which may destroy the amplifier.
In addition, if the output of the RF signal exceeds the withstand voltage of an antenna switch due to factors such as fluctuations in battery voltage, fluctuations in output load, or fluctuations in temperature, a power drop may occur. The occurrence of a power drop is due to leakage of current from a field-effect transistor in the antenna switch.